The network-info package

This library provides simple read-only access to the
local computer's networking configuration. It is
currently capable of getting a list of all the network
interfaces and their respective IPv4, IPv6 and MAC
addresses.

network-info has been tested and is known to work on
Ubuntu 10.10, FreeBSD 9.0, Mac OS X 10.6.4 and Windows
XP/7. It probably works on other flavours of Linux,
OS X, FreeBSD and Windows as well.